A two-ion fluid model with gyrotropic pressures for the comet interaction with the solar wind

A two-ion fluid model (with gyrotropic pressure tensor) for the interaction of cometary pickup plasma with the solar wind has been developed. Whereas previous models have, in general, assumed a single fluid plasma, this generalized model follows the evolution of two fluids - solar wind protons and cometary ions. The two fluids are coupled by the magnetic field and by coupling terms which are the velocity moments of the time relaxation model of Bhatnagar-Gross-Krook. Furthermore, a gyrotropic pressure tensor replaces the isotropic pressure assumed in other models. Isotropization terms couple the pressures in the directions perpendicular and parallel to the magnetic field. A Godunov type upstream method which uses an approximate Riemann solver adapted to the model equations is presented.